A hard disk apparatus for use in a personal computer, a car navigation and so forth is equipped with a spindle motor for rotating a magnetic disk about a center axis thereof. The spindle motor is of a configuration in which a rotor is rotated relative to a stator through a bearing device. In recent years, a fluid dynamic bearing device with lubricating oil filled between a shaft and a sleeve is widely used as a bearing device for the spindle motor. The fluid dynamic bearing device allows the shaft and the sleeve to make relative rotation, while generating a fluid dynamic pressure in the lubricating oil by use of radial dynamic pressure grooves formed in an outer circumferential surface of the shaft or in an inner circumferential surface of the sleeve and thrust dynamic pressure grooves formed in one end surface of the shaft or in one end surface of the sleeve.
Conventional fluid dynamic bearing devices are disclosed in, e.g., Japanese Patent Laid-open Publication Nos. H09-044985 and H11-117934.
Some of the conventional fluid dynamic bearing devices are of the type including a sleeve made of a porous sintered body and a bearing housing surrounding the sleeve to prevent leakage of lubricating oil. As the bearing housing, there is used a bottom-closed cylindrical housing produced by press-forming a galvanized steel sheet (SECE) obtained by electro-galvanizing, e.g., a cold-rolled steel sheet (SPCC, SPCD, SPCE or the like). Ester oil superior in flowability and thermal stability is often used as the lubricating oil.
However, since zinc contained in a galvanized layer of the bearing housing acts as a catalyst against the ester oil, there is a possibility that the ester oil in the bearing housing may be changed in quality and deteriorated by the zinc. FIG. 8 is a graph representing the distribution of absorbance of the ester oil against lights of different wave numbers irradiated on the ester oil, which is measured by use of a Fourier transform infrared spectrophotometer (FT-IR). FIG. 9 shows same as FIG. 8, but the distribution is measured by use of a Fourier transform infrared spectrophotometer (FT-IR) after zinc is added to the ester oil and a few days has lapsed. In FIGS. 8 and 9, the horizontal axis denotes the wave number of the light irradiated, while the vertical axis stands for the absorbance. Comparison of FIGS. 8 and 9 reveals that, in FIG. 9, the absorbance is decreased at the wave number around 1596/cm (the circled portion “A” in FIG. 9). In other words, it can be seen that the molecular bonds in the ester oil have undergone certain modification under the action of zinc and the ester oil has suffered from a change in quality.
On the other hand, if a non-galvanized cold-rolled steel plate is used as a material of the bearing housing, it is possible to avoid the change in quality of the ester oil which would otherwise be caused by zinc. Even in this case, however, there is a possibility that the composition of the ester oil may be affected by metallic components mixed into the lubricating oil during the sliding movement of the bearing housing relative to the member received in the bearing housing or by rust generated on the surface of the bearing housing.
If high-priced resin (LCP resin or PPS resin) or stainless steel, which is less likely to change the quality of the ester oil, is used as a material of the bearing housing, the cost of producing the bearing housing is increased, thus hindering cost-effective production of the fluid dynamic bearing device.